Accommodation, pupilloconstriction and choroidal blood flow are all clinically important ocular functions. Both the nature of the visual stimuli that elicit each of these responses and the peripheral mechanisms by which each is produced have been the subject of investigation. Yet surprisingly little is known about the central neural circuits involved. The delineation of the anatomical pathways mediating accommodation, pupilloconstriction and choroidal blood flow and the possible sites of interaction between them would make it possible to test a variety of fundamental hypotheses concerning the respective roles of these three ocular functions in a variety of clinical pathologies. The long-term goal of this research program is to improve our understanding of the functional organization of accommodation pathways that control the ability to focus the eye in vertebrates and use this information to determine the role that the central nervous system may play in the development of myopia. Over the past decade, the chick has become an important model for the study of myopia. In the proposed experiments, chronic stimulation of accommodation "centers" in the developing chick brain will be used to test directly the clinical "near work" hypothesis that overaccommodation, or too much near vision, can produce myopia or nearsightedness. In addition, using pathway tracing techniques in conjunction with microstimulation, the brainstem circuits involved in accommodation will be determined. In summary, this set of experiments will anatomically and functionally define the brainstem circuitry controlling accommodation and determine its role in the causation of experimental myopia.